Reference is made to commonly assigned U.S. patent application Ser. No. 09/832,759 filed Apr. 11, 2000 by Keith B. Kahen et al. and U.S. patent application Ser. No. 10/066,936 filed concurrently herewith entitled xe2x80x9cOrganic Vertical Cavity Lasing Devices Containing Periodic Gain Regionsxe2x80x9d by Keith B. Kahen et al.; the disclosures of which are incorporated herein by reference.
The present invention relates to an organic vertical cavity laser light producing device.
Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) have been developed since the mid-80""s (K. Kinoshita et al., IEEE J. Quant. Electron. QE-23, 882 [1987]). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (K. D. Choquette et al., Proc. IEEE 85, 1730 [1997]). With the success of these near-infrared lasers in recent years, attention has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (C. Wilmsen et al., Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many fruitful applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (T. Ishigure et al., Electron. Lett. 31, 467 [1995]). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) which span the visible spectrum.
In the effort to produce visible wavelength VCSELs, it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size, and most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip.
The usual route for making a manufacturable laser diode system is to use electrical injection rather than optical pumping to create the necessary population inversion in the active region of the device. This is the case for inorganic systems since their optically pumped thresholds (P. L. Gourley et al., Appl. Phys. Lett. 54, 1209 [1989]) for broad-area devices are on the order of 104 W/cm2. Such high power densities can only be achieved by using other lasers as the pump sources, precluding that route for inorganic laser cavities. Unpumped organic laser systems have greatly reduced combined scattering and absorption losses (xcx9c0.5 cmxe2x88x921) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. As a result, optically pumped power density thresholds below 1 W/cm2 should be attainable, especially when a VCSEL-based microcavity design is used in order to minimize the active volume (which results in lower thresholds). The importance of power density thresholds below 1 W/cm2 is that it becomes possible to optically pump the laser cavities with inexpensive, off-the-shelf, incoherent LED""s.
In order to produce single-mode milliwatt output power from an organic VCSEL device, typically it is necessary to have the diameter of the emitting area be on the order of 10 xcexcm. As a result, 1 mW of output power would require that the device be optically pumped by a source producing xcx9c6000 W/cm2 (assuming a 25% power conversion efficiency). This power density level (and pixel size) is far beyond the capabilities of LED""s and, additionally, would most likely cause some degradation problems with the organic materials if they were driven cw. A path around that problem is to increase the organic laser""s emitting area diameter to around 350 xcexcm, which would reduce the pump power density level to 4 W/cm2 (to produce 1 mW of output power). This power density level and pixel size is achievable by off-the-shelf 400 nm inorganic LED""s. Unfortunately, broad-area laser devices having 350 xcexcm diameter emitting areas would lead to highly multimode output and to lower power conversion efficiencies (as a result of filamentation). As a result, it is highly advantageous to produce large area organic VCSEL devices, which have good power conversion efficiencies and single-mode outputs.
It is an object of this invention to provide an organic surface emitting laser arrangement that is particularly suitable to permit phase-locked laser emission from a two-dimensional array of micron-sized organic laser pixels.
These objects are achieved by an organic vertical cavity laser array device, comprising:
a) a bottom dielectric stack reflective to light over a predetermined range of wavelengths;
b) an organic active region for producing laser light;
c) a top dielectric stack spaced from the bottom dielectric stack and reflective to light over a predetermined range of wavelengths; and
d) an etched region formed in a surface of the bottom dielectric stack to define an array of spaced laser pixels which have higher reflectance than the interpixel regions so that the array emits coherent phase-locked laser light.
It is an advantage of the present invention to provide two-dimensional organic laser array devices employing micron-sized laser pixels which can be either electrically or optically driven by large area sources and produce phase-locked laser output. The devices employ a microcavity design incorporating high reflectance dielectric stacks for both the top and bottom reflectors; and have a gain media including small-molecular weight organic material. The micron-sized laser pixels of the device are provided by modulating the reflectance of the bottom dielectric stack. The emission from the pixels is phase-locked, which enables the device to be driven by a large area source while the laser output remains single-mode (or at most two lateral modes). Combining low power density thresholds with pumping by large area sources enables the devices to be optically driven by inexpensive incoherent LED""s.